Tone controlled wireless telephone extension system



J. P. DRIVER Jan. 30, 1968 TONE CONTROLLED WIRELESS TELEPHONE EXTENSIONSYSTEM 4 Sheets-Sheet l Filed Nov. 8, 1965 J. P. DRIVER 3,366,880 TONECONTHOELED WIRELESS TELEPHONE EXTENSION SYSTEM 4 Sheets-'Sheet vS3 Jan.30, 1968 Filed Nov. a, 1965 A im@ l. F. DRIVER Jan. 30, 1968 TONECONTROLLED WIRELESS TELEPHONE EXTENSION SYSTEM 4 Sheets-Sheet 5 FiledNov. 8, 1965 .omo mzOP Q24 .QmO E OP J. P. DRIVER Jan. 30, 1968 TONECONTROLLED WIRELESS TELEPHONE EXTENSION SYSTEM 4 Sheets-Sheet 4 FiledNov. 8, 1965 United States Patent Oce 3,366,380 Patented Jan. 30, 19683,366,880 TONE CGNTROLLED WIRELESS TELEPHGNE EXTENSION SYSTEM Joel P.Driver, Las Vegas, Nev., assigner to American Telephone ExtensionCorporation, Las Vegas, Nev., a corporation of NevadaContinuation-impart of application Ser. No. 272,587, Apr. 12, 1963. Thisapplication Nov. 8, 1965, Ser. No. 566,784

21 Claims. (Cl. S25-22) The present invention relates in general to acommunication system for relaying information between two stations and,more particularly, to a system which is adapted to act as a remotetelephone extension. This application is a continuation-impart of myprevious application S.N. 272,587, filed Apr. 12, 1963 and nowabandoned.

In many circumstances, and particularly in the case of businesses whereit is frequently necessary to be in the eld and out of the reach of atelephone, it is desirable to establish a wireless contact with a homeor oice for the purpose of maintaining communication therewit-h. In sucha situation, the advantages of bein'3 able to receive incoming andinitiate outgoing calls from the eld are obvious. Heretofore, it hasbeen difficult to establish such direct contact except through complexand expensive circuitry which made such equipment impractical.

Many attempts have been made, with varying degrees of success, to devisesuch a system which would be compact, simple of construction, reliable,and easy to operate. The present invention provides such a system.Basically, the present invention provides a base station unit, forinter-connection with a telephone system, and a portable remote unitwhich acts as a wireless extension telephone. Both units aretransceivers, Abeing designed to perform both transmitting and receivingfunctions. Since both units are capable of both functions, it will beapparent that the units have use not only as telephone extensions, butas general communications relay links as well.

In one embodiment, the base station described herein is used as a relaylink between a telephone system and a remote location. The telephonesystem is coupled to the base station by either a telephone line or anintermediate transmission line, and, when activated by audio or ringingsignals, the base station will switch from its normal receive mode totransmit audio and ringing signals by way of a radio frequency (RF)carrier to a remote station, At the end of each transmission, the basestation reverts to its receive mode.

The remote transceiver, normally in the receive mode and tuned to theproper RF frequency channel, receives the ringing signal and produces anaudible ringing sound. The remote transceiver may be a portable,hand-held unit of low range, or a larger mobile unit wit-h a longerrange.

Upon receipt of a ringing signal at the remote station, which may be oflthe press-to-talk type, or its voice-operated equivalent, the handsetis picked up and the talk button pressed to switch the remote unit tothe transmit mode. The remote unit is provided with a coding toneoscillator which modulates the transmitted RF carrier at a frequencypreferably above the audio range, and may also be provided with a dialmechanism to permit calls tofbe originated at the remote unit.

Upen receipt of the RF `carrier and tone from the remote transceiver,the base station transceiver unit is held in its -receive mode and theremote unit is connected to the telephone system through the receiversection of the base station. This effectively answers the telephone fromthe remote extension. Means are provided to hold the base station in thereceive mode in response to and during reception of the tone from theremote unit, thus permitting continued transmission -of signals to thetelephone system.

Upon release of the talk button at the remote unit, the remote unitswitches to receive and releases the base unit, which `again ispermitted to switch to its transmit mode in response to audio signalsfrom the telephone system. In this manner, two-way conversations may beconducted between a telephone subscriber and the operator of the remote.wireless telephone extension.

Not only is the base unit useful as a. means for esta'blis'hingconnections between la mobile remote extension and a telephone system,but it also has utility, when combined with a conventional transceiver,as a relay link between a base station and a remote station and thus canextend the range of the system, provide coverage of .a normally deadarea, act as a frequency shifter, or perform any of the tasks normallyassigned to radio relay links. yIn addition, the combination may be usedto relay information from a first rem-Ote mobile station to a secondremote mobile station. When used as a radio relay link in this manner,signals may `be received from, eg., a remote station by the receiversection of the relay unit of the invention, transferred to the standardtransceiver and transmitted thereby `at a second frequency. Similarly,signals sent from, eg., a base station may 'be picked up by the standardtransceiver, transferred to the relay unit off the invention, andtransmitted thereby at a second frequency.

The system .according to the present invention has been found to haveparticular utility in the range of radio frequencies known as theCitizens Band. However, the tremendous popularity of this frequency'band causes a great deal `of crowding of the channels, and presentsproblems in preventing spurious signals ifrom interfering with operationof the telephone extension. -In order to overcome such interference andto make 'telephone calls secure 4from undesired monitoring, theinvention incorporates a coding tone modulation with `both the base andremote units being responsive only to carrier waves modulated by codedtone signals having a predetermined characteristic. This enables thesub-ject system to reject signa-ls which do not carry the selectedt-one, and thus avoid interference. As an additional benefit, the use ofsuch a coded tone `modulation makes it possible to operate a number oftelephone extension systems on a single carrier frequency channel, eachsystem being sensitive only to a predetermined tone pattern and thussecure `from interference from other systems operating on the samechannel, but using different tone patterns. The use of this tone codingalso makes possible the use of several remote units with a single baseunit, or vice versa, the several units using the same code as the baseso as to be responsive only to that one 'base unit. An object of thepresent invention is, therefore, the provision of a communication systemfor relaying signa-ls between two stations which, lby reason of its useof solid state components, is compact in size, requires small powerconsumption, is reliable and fast in operation.

Another object of the invention is the provision of a communicationsystem which enables radio coverage to be `obtained in dead areas or atdistances normally out of range of each other.

A further object of this invention is the provision of a transmissionand receiving system for use with an additional transceiver as awireless radio relay between two remote stations.

It is an object of this invention to provide a communication system foruse as an answering extension whereby a telephone may be answered at alocation remote from the telephone instrument in response to a signalfrom a base station and whereby a two-way conversation can be carried onbetween the telephone system and the remote station.

It is an additional object of the invention to provide a communicationsystem for use as a telephone extension wherein telephone calls can beinitiated at the remote station, said remote station being provided witha dial mechanism which may be used to dial any desired telephonesubscriber.

Another object of the invention is the provision of a remote telephoneanswering device linked to a cornmercial telephone system by a two-wayradio system utilizing a push-to-talk or equivalent voice-operatedremote unit. The device so provided includes means which permit atwo-way conversation to be carried on without interruption by spurioussignals which create the impression that the remote unit has terminatedthe conversation, but which automatically hangs up the telephone at theend of the conversation.

It is another object of the invention to provide a comf municationsystem which is capable of operating on a shared frequency channel, butwhich will respond only to predetermined signal sources.

lt is a further object of the invention to provide a communicationsystem which includes a coded tone remote telephone extension which iscompact and easily portable, but which permits two-way communicationwith a commercial telephone system through a base station.

A further object of the invention is the provision of a remote telephoneextension transceiver having a pushto-talk mode of operation or itsequivalent and including coding means to permit shared-channeloperation, the remote unit being coded so as to be operable only througha predetermined base station.

Additional objects, features and advantages of the invention will beapparent from the following detailed description of a preferredembodiment thereof, selected for purpose of illustration and shown inthe accompanying drawings, in which:

FIG. l shows in block diagram form a preferred embodiment of a telephoneanswering system made in accordance with the present invention;

FIG. 2 is a schematic diagram of the circuitry for coupling the systemto a telephone system;

FIG. 3 is a schematic diagram of the antenna control circuitry;

FIG. 4 is a schematic diagram of a modification of the base station unitwhich permits its operation in a wireless radio relay link;

FIG. 5 is a block diagram of a preferred embodiment of a remotetelephone answering device made in accordance with the presentinvention; and

FIG. 6 is a diagrammatic illustration of the use of the present systemin a radio relay link.

Referring now to the block diagram of the system in FIG. l, there isillustrated a communication system providing an answering extensionwhich permits a two-way conversation between a base telephone system anda remote station. Although illustrated as a remote telephone extensionsystem, it will be apparent that the basic circuitry shown in thisfigure is easily adaptable for use as a radio relay link.

The communication system comprises a base transceiver 10 and a remotetransceiver 12. Base transceiver 10 comprises a transmitter 14 and areceiver 16 each connectable through an antenna switch 18 to an antenna20. The base transceiver 10 is normally in its receive mode, i,e.,antenna 20 normally is connected through antenna switch 18 to thereceiver 16, but is switchable to the transmit mode under the control ofa monostable fiip-flop 22 and a flip-flop follower switch 24, theflip-flop 22 being switched to its unstable condition to place thetransceiver in its transmit mode. This disconnects receiver 16 andconnects antenna 20 through antenna switch 18 to the transmitter 14. Thetransceiver remains in its transmit mode only as long as there is anaudio frequency signal present to hold flip-flop 22 in its unstablecondition.

The transceiver is capable of receiving and transmitting radio frequencysignals through antenna 20 and receiving and sending out audio frequencysignals by way of a transmission line 26. When the system of theinvention is used as a telephone extension, the transmission line 26 maybe coupled to a commercial telephone system 28, the connection beingmade through the circuitry of a telephone receiver located at or nearthe base station. This connection is an electrical coupling, bypassingthe telephone receiver handset and permitting direct communicationthrough the telephone receiver circuitry to the telephone system, andthus through the central ofiice of the system to other systemsubscribers. When the communication system of the invention is used as arelay link, the transmission line 26 of the transceiver may be used toconnect transceiver 10 to a second, conventional (i.e., uncoded)transceiver or it may be omitted. In the latter case, the output signalfrom transceiver 10 which is to be retransmitted is obtained directlyfrom the audio stage of its receiver portion, while the input signalwhich is received from the second transceiver is applied to the audioinput of the transmitter portion of the transceiver 1t). In eitherusage, the radio frequency signals may be directed to and received froma remote transceiver such as the small, hand-held unit 12. Transmissionline 26 is shown Iconnected through a dial-out puiser 30, the functionof which will be described in detail below, to an impedance matchingnetwork 32. Signals applied to the transceiver through transmission line26 are then lfed through line 34 to the audio amplifier 36 oftransmitter section 14. These signals are also fed through line 38 to avox amplifier 40 where they are amplified and fed through a volta-gedoubler 42 to the flip-iiop control 44.

Transmitter 14 is a conventional, preferably transistorized,information-modulated transmitter having, in addition to audio amplifier36, a modulator 46, RF oscillator 48 and an RF amplifier S0. Alsoincluded in the transmitter is a tone code oscillator 52 whichsuperimposes a coding signal of any desired pattern or characteristic onthe audio signals applied to audio amplifier 36 for transmissiontherewith. The oscillator 52 may provide a continuous tone signal of afrequency within the audio range or outside it, or may produce codedpulses of any known type, as, for example, digital pulses in timedrelationship with a synchronization signal.

RF signals received by antenna 2t) are connected through antenna switch18 to the receiver section of the transceiver. Receiver 16 is aconventional, preferably transistorized, superheterodyne receiver havingan RF amplier 60, a mixer 62, a local oscillator 64, an IF amplifierstage 66, a first detector 68, and first and second audio amplifiers 70and 727 all connected in the well-known manner. The first audioamplifier 70 is normally in an off condition and remains in thiscondition until it is turned on by a control signal obtained in responseto the tone coding signals on the input carrier signal. This controlsignal is derived from detector 68 and is carried by line 74- to afrequency sensitive filter 76. Filter 76 is responsive only to signalsof a predetermined code pattern, thus providing the base transceiverwith a highly selective response to input signals. The output of filter76 is amplified in filter amplifier 78 and is converted to a DC voltagein the voltage control circuit 80. The resulting DC voltage is amplifiedby DC amplifier 82 and passed to the on-ofi circuit `84. This circuitresponds to its input, which may be a continuous or a pulsating DCcurrent, to provide three control signals. The first control signal isapplied through line 56 to the first audio amplifier 70 of receiver 16to turn it on, enabling it to pass audio signals to the second audioamplifier 72. The second control signal is applied through line 88 to ananti-vox circuit 90 which produces a signal on line 92 to disable thevox amplifier 40 and to prevent the transceiver 10 from switching to thetransmit mode. This disabling voltage remains on line 92 as long as aninput signal which carries the coding signals to which the frequencysensitive filter 76 is responsive is received at antenna 20. The thirdcontrol signal provided by on-off circuit 34 is fed through line 94` toa time delay circuit 96 which immediately activates a relay RL1 to closeits associated contacts 98, completing the circuit from audio amplifier72 of receiver 16 to the impedance matching circuit 32 and thus to thetransmission line 26.

To accommodate the use or a dial mechanism at a remote station such asthe remote transceiver 12, the dialout pulser 30 is provided tointerrupt the connection between the impedance matching circuit 32 andthe transmission line 26 or to provide touch tone coding for telephonesystems adapted for this type of coding. This interruption is inresponse to and corresponds to a pulsed coding signal received atantenna 20, which signal actuates `the ori-off circuit S4 and thus theanti-vox circuit 90 to produce a pulsating signal on line 100. Thispulsating signal, when applied to the dial-out circuit 30, produces thedesired interruption of the connection to effect a dial-out operation.

The remote transceiver 12 includes a receiver section 102 and atransmitter section 104 connectable through a switch 106 to an antenna10S. Switch 106 may be of the push-to-talk type or may be avoice-operated switch. The transceiver 12 includes a speaker-microphone110 which is normally connected to receiver 102 and acts as aloudspeaker. When the push-to-talk button is depressed, the speaker isconnected through a modulator section 112 to the transmitter so thatvoice transmission can occur in the normal manner. It will be apparentthat a separate microphone and loudspeaker can be provided. Superimposedon the audio signal applied to modulator 112 is a coding signal from acoding or tone oscillator 114. A dial mechanism, illustratedschematically at 116 by a rotating cam and normally-closed switch 118,pulses the code signals applied to the modulator 112 to provide thenormal dialing signals required by commercial telephone systems. Theresulting pulsed tone signal may then be transmitted by the remotetransceiver 12 for reception at the base station, where the signal isconverted to the type of signal required by the telephone system.

When the base transceiver is connected to a telephone system 28 by meansof the T-line 26, a system subscriber wishing to call the telephone towhich the transceiver 10 is connected at the base station will dial itsnumber in the usual manner, sending ringing signals through the T-line26 to lthe impedance matching circuit 32. The ringing signals lpassthrough the impedance match 32 to vox amplifier 40, voltage doubler `42and flip-fiop control 44. The output of the flip-flop control causes themonostable flip-flop 22 to shift to its unstable state of conductionand, in turn, causes the fiip-lio-p follower 24 to change its starte ofconduction thereby to turn on transmitter 14 and to connect it to theantenna, The ringing signal on the T-line is also supplied by way ofline `34 to audio amplifier 36 and is ultimately transmitted from an-Itenna 20. Modulating the transmitted RF signal is the coded output ofthe tone oscillator 5 which is combined with the aud-io frequencysignals applied to the transmitter. The tone oscillations may be off anysuitable frequency, either in the audio range or above it, and may be acontinuous wave oscillation or may be a pulsed signal having a suitablepattern. During any pauses in the signal to be transmitted, and at itscompletion, the monostable flip-flop 22 automatically reverts to itsstable state of conduction. This reconnects the antenna 20 to the RFamplifier 60 of receiver 116 and turns the receiver on by applying avoltage to the local oscillator 464 and the IF amplifier 66. The basetransceiver is thus in condition for receiving a signal from remotetransceiver 12 during such pauses, allowing the remote unit to controlthe direction of transmission. Through the use of semiconductor devicesin this system, the switching time is very short, allowing thetransceiver to switch between its transmit and receive modes even duringextremely brief pauses in the input audio frequency signal.

Remote transceiver 12 is normally in the receive mode; any signals sentout by the base transceiver 10 are intercepted by antenna 108 and, ifproperly coded, are fed through receiver section 102 to the loudspeaker110. If a ringing signal has been transmitted by the base unit, theoutput from the loudspeaker will be an audible ringing sound. To answerthe telephone, the push-to-talk button on the remote transceiver ispressed, switching the remote transceiver to its transmit mode andcausing the loudspeaker to act as a microphone if a single transducer isused. Upon connection of antenna 108 to the transmitter section |104 ofthe remote unit, a carrier frequency is immediately transmitted.Modulating the `carrier signal will be a coding signal from the toneoscillator 114, applied to the transmitter section by way of modulatorsection 112 and thus combined with any audio signals from microphone110.

The answering transmission from the remote transceiver 12, whichincludes an RF carrier signal and a tone oscillation, and which may alsoinclude a voice modulation, is received by the base transceiver 10 atthe antenna 20. As soon as the base transceiver reverts to its receivemode, as at the end of a ringing signal,l the R-F input to antenna 20passes through the antenna switch 18 to RF amplifier 60 in the receiversection. The RF signal is removed in the receiver in the normal mannerand the audio output signal due to voice modulation, if any, is appliedto the audio amplifier 70. Filter means (not shown) may be provided toseparate the coding signals and the audio frequency signals between thedetector and amplifier 70. Such a filter would be needed if the codingsignals are in the audio frequency range to which telephone systems canrespond, i.e., in the range of 250 to 3000 cps. Signals outside thatrange need not be filtered, for the telephone system will not pass suchsignals and will thus act as a filter. The audio frequency signals andthe tone signals are obtained from detector 68 and applied through line74 to the frequency sensitive filter 76 which responds only to codingmodulations of a predetermined frequency or pattern to provide an outputsignal. The output signal serves to turn audio amplifier 70 on, todisable vox amplifier 40 and to close contacts 98 in the above-discloseddescribed manner. As long as the pushto-talk button 106 or itsequivalent on the remote transceiver is held down, the base transceiverwill remain in its receive mode with the vox amplified disabled, theaudio amplifier 70 turned on and relay RL1 energized to hold contacts 98closed. Contacts 98 effectively pick-up the telephone receiver at thebase station, so that audio signals can pass into the conventionaltelephone system. Upon release of the push-to-talk button, the tonesignal will be removed from the on-off circuit 84, turning off theanti-vox signal and the audio amplifier 70 to permit the basetransceiver to be switched to its transmit mode. By reason of the timedelay circuit 96, however, relay contacts 98 remain closed for asuitable length of time, for example, thirty seconds. This delaymechanism holds the telephone in a picked-up condition even aftertransmission from the remote station has stopped, thus avoiding theappearance that the telephone receiver has been hung-up every time thepush-to-talk button 106 at the remote unit is released. However, thedelay circuit does not hold the base station in its receive mode, butpermits it to be switched to its transmit mode under the control ofsignals from the T-line 26.

By a repetition of the foregoing sequence, a two-way conversation `maybe carried on between the subscriber at the telephone system 28 and theperson answering the remote telephone extension transceiver 12, with thedirection of conversation being under control of the remote unit.

As has been mentioned above, the remote unit may be provided with a dialmechanism of a suitable type to permit calls to be initiated from theremote extension. To initiate such a call the push-to-talk button on theremote unit is pressed to place the ltransceiver in its transmit mode.The RF carrier and the coding modulation are transmitted to the basetransceiver, locking it in its receive mode by energizing the anti-voxcircuit 90 as described above, and connecting the audio amplifier 72YYto the impedance Ymatching .circuit V32. TheV dial mecha-n nism 115 isthen operated to open and close contact points 118, modulating theoutput of the tone oscillator into a series of pulses or providing othersuitable signals, as required by the telephone companys central system.These pulses are received by the base transceiver 10 and are appliedthrough the frequency sensitive filter 76 to the Von-off circuit 84.These pulses are then fed through line 88, anti-vox 90 and line 100 tothe dial-out pulser 3f) which responds to each pulse to interrupt thecompleted DC circuit between audio amplifier 72 and the telephonesystem. The opening and closing of this circuit places pulses on theT-line and thence into the telephone system. Such pulses are identicalto those normally applied to a telephone line by a dial mechanism andthus operate in a similar manner to dial the desired subscriber number.'In telephone sytsems which utilize the touch tone type of dialing, thedial-out pulser circuit 30 may be modied to provide the type of signalrequired to effect a dial-out operation.

If the base transceiver 16` is to be used as a communication relay linkas, for example, in applications other than telephone extension systems,it will be apparent that only minor modifications need be made. In sucha situation, line 26 would be connected to a third but standardtransceiver unit such as, for example, a commercially available citizensband mobile transceiver. A tone code oscillator 52 would not be added tothe standard transceiver if its bandpass were sufficiently broad to passthe coding signals. Time delay circuit 96 would Ibe modified toeliminate its delay characteristics and relay RLl would be modified tooperate the transmit-receive switch of the standard transceiver. Whenthe transceiver 10 portion of the relay unit is receiving, the codingand audio signals will be transferred through T-line 26 to the standardtransceiver and transmitted thereby. Similarly, signals received by thestandard unit will be applied through T- line 26 to the transceiver 19portion of the relay unit and transmitted thereby. The two transceiverswill be operating in opposite modes and normally would operate ondifferent frequency channels. If such a relay unit were operating inconjunction with a telephone extension system, intermediate the baseunit att-ached to the telephone systern and the remote transceiver 12,the tone coding normally would be passed through the relay link alongwith the audio frequency signals.

Turning now to a more detailed description of the circuitry used in apreferred embodiment of the subject system, reference is made to FIGS. 2and 3 which show the construction of the base transceiver 10.

Again considering the system from the point of view of a signal arrivingfrom telephone system 28 and applied to transmission line 26, such asignal is applied by way of line 26 to the impedance matching circuit 32through the normally-closed contacts 120 in pulser circuit 30. Theimpedance match is shown as a transformer T11 having a DC blockingcapacitor C63 serially connected with the primary winding and connectedacross transmission line 26. Capacitor C63 serves to block from T11 theDC voltage that appears on line 26 when connected to a telephone system.The capacitor is of such a value as not to affect the audio frequencysignals which are coupled to the secondary winding of transformer T11and pass by way of line 34 to the audio amplifier 36 of the transmitter14 and by way of line 38 to vox amplifier 40 (FIG. 3).

A coupling capacitor C51 connects the secondary winding of transformerT11 to the input of vox amplifier 40, vi.e., to the Ibase of atransistor Q1. Vox amplifier 40 is comprised of transistor Q1 havingbias resistors R85 and R86 connected between the base electrode andground and between the base electrode and the direct current voltagesupply B-, respectively. A bias resistor R88 is connected between theemitter and the voltage supply ,andris bypassed by a capacitor C53.Variable resistor R87 is connected between? the collector of l(ransistorQIV and ground. The output of transistor Q1 is fed through the tap onvariable resistor R87 through a coupling capacitor C52 to the base of asecond transistor Q2. Transistor Q2 similarly has a bias resistor 89connected between the base and ground and a bias resistor R90 connectedbetween base and the Voltage supply. A parallel RC network comprisingresistor R91 and capacitor C54 is connected between the emitter oftransistor Q2 and the supply B-. The collector of Q2 is connected to theprimary winding of a coupling transformer T10. Resistance R87 andcapacitance C51 are so selected that the signal applied to line 26 willnot cause transistor Q2 to conduct unless there is an audio frequencymodulation. The modulated signal produced at the base of Q2 is thenamplified by transistor Q2 and passed through transformer T19. Thissignal is rectified by a voltage doubling circuit 42 which is composedof diodes 130 and 132 and capacitors C55 and C56 connected in awellknown manner. The output of this doubling circuit is coupled byresistance R92 to the base of a transistor Q3 in the flip-Hop controlcircuit 44. The collector of transistor Q3 is connected to ground andthe emitter is connected through a load resistance R93 to voltage supplyB-. Transistor Q3 is, in effect, an emitter-follower switch, which, uponconduction, connects line 134 to ground potential throughserially-connected diodes 136 and 138. Line 134 is connected throughisolation resistor R59 to the base of the normally non-conductivetransistor in fiipflop circuit 22. Diodes 136 and 138 form a singledirection current path to limit the conduction of current between thebase electrode of the non-conductive transistor in the flip-dop andground when transistor Q3 is conductive.

Flip-flop 22 is a monostable multivibrator including a normallyconductive transistor Q4 and a normally nonconductive transistor Q5connected in a known manner. The collector electrode of transistor Q4 isconnected through resistor R52 to the base of transistor Q5, applying avoltage thereto to maintain it in a non-conductive state. The collectorof transistor Q5 is similarly connected through a resistor R53 to thebase of transistor Q4. Since Q5 is not conductive, no cut-off voltage isapplied to the base of Q4. Bias resistors R57 and R58 are connectedbetween the base of Q4 and the base of Q5, respectively, and the voltagesupply 2B-. A load resistor R51 is connected between the collector of Q4and ground while a resistor R54 is connected between the collector of Q5and ground. Load resistors R50 and R55 are connected between thecollectors of Q4 and Q5, respectively, to their corresponding followerstages in the ip-op follower 24.

Application of a ground potential to the base of transistor Q5 by meansof the conduction of transistor Q3 removes the cut-off voltage from Q5and permits it to conduct, thus causing a cut-off voltage to appear atthe base of Q4. This causes the monostable circuit to begin conductingin its unstable condition, in which state it will remain untiltransistor Q3 is cut off.

The Hip-flop follower 24 consists of a pair of transistors Q6 and Q7whose states of conduction are opposite to the states of conduction oftheir respective control transistors. The base of transistor Q6 isconnected to the collector of transistor Q4 and thus is normally cutoff.The base of Q7 is connected to the collector of transistor Q5 and thusis normally in a conductive condition. The base of Q6 is connectedthrough resistor R45 to the collector of Q7 and through resistor R48 toa source of bias voltage. Similarly, the base of Q7 is connected throughresistor R46 to the collector of Q6 and through resistor R49 to thesource of bias voltage. The collectors of the respective transistors infiip-fiop follower 24 are connected to the antenna switch 18 to controlthe connection of antenna to one or the other of transmitter 14 andreceiver 16.

The collector of transistor Q7, which is normally conductive, isconnected through the primary Winding Fl`3-P of the coupling transformerwhich connects the antenna switch to the receiver 16, through a variableloading coil L2, a diode 140, an isolation coil 142, and a resistor R43to ground. The iiow of current through this path by reason of theconduction of transistor Q7 unblocks diode 140 and permits antenna 20 to`be coupled through capacitor C61 and primary winding T3-P to thereceiver.

The collector of Q6 is connected through the secondary winding T4-S ofthe output transformer of transmitter section 14, a variable loadingcoil L1, a diode 144, coil 142 and resistor R43 to ground. Antenna 2t]is connected through capacitor C61 to the junction of backto-back diodes144 and 1140. The collector of Q6 is also connected across a resistorR44 to the input of RF oscillator 48 and to the input of tone codeoscillator 52 in transmitter section 14 to turn on these circuits.Similarly, the collector of transistor Q7 is connected across a resistorR47 to the inputs of the local oscillator 64 and the RF amplifier 66 ofreceiver section 16 to turn these circuits on.

As has been noted above, transistor Q7 is normally conductive to providea current path through diode 140 and effectively to connect antenna 2t)to the receiver section of the hase transceiver. At the same time, theconduction of Q7 applies the supply voltage to the local oscillator andIF amplifier of the receiver section to allow them to operate. When anaudio frequency signal such as a ring or voice modulation passes throughvoX amplifier 40, voltage doubler 42 and causes transistor Q3 of thefiip-iiop control circuit to become conductive, a ground potential isapplied to the ybase of Q5, as previously described. The conduction ofQ5 changes the bias on the base electrode of transistor Q7 and causes itto stop conducting. This cuts off the current path for diode 140 between ground and the supply voltage and also removes the supply voltagefrom the receiver section circuits. When diode 140 stops conducting, theantenna is disconnected from the receiver and the receiver is turnedoff.

The switching of flip-Hop 22 to its unstable state causes transistor Q4to stop conducting, changing the bias voltage at the base of followertransistor Q6 and permitting that transistor to start conducting. Theconduction of Q6 opens a current path between the bias supply voltageand ground through diode 144, causing diode 144 to conduct and thus toconnect antenna 20 to the transmitter section 14. Conduction oftransistor Q6 also applies the bias voltage to the oscillator circuitsof the transmitter and thus turns the transmitter on. Since the responsetime of the antenna switch is very short, the audio signal appearing atthe transmission line 26 is immediately transmitted with no detectableclipping.

Inasmuch as the flip-fiop 22 is of the monostable type and will remainin its unstable condition only so long as a ground potential is appliedto the base transistor Q5, the system will remain in the transmit modeonly so long as an audio signal is applied to the transmission line.Upon any pause or interruption in the audio signal the flip-op willrevert to its initial condition and will return the system to thereceive mode.

Considering now the portion of the base transceiver 10 which isoperative during the receive mode, input signals from the antenna 20 areconnected through the antenna switch 18 to the receiver 16, as abovedescribed. The coding signal and any audio frequency modulations areapplied through line 74 to the input of frequency sensitive filter 76(FIG. 2). The filter network includes a branch, comprising resistorsR60, R61, R62 and capacitors C33, C34, and C35 connected in. 1a mannerwell known in the filter art, having a design and bandwidth sufiicientto pass the coding signals, which have a predetermined characteristic.The signal from the detector 68 is applied to this branch, here shown asa high-pass lter designed for tone frequencies above audio, through acapacitor C32 and thence to the base of a transistor Q8. In theillustrated embodiment a special high-gain tone filter comprised of anRC double-T filter with resistors R63, R65, R66 and capacitors C38, C39and C40 connected in a feed-back arrangement across the collector andbase of transistor Q8 in a manner similar to a phase shift oscillator isprovided. A variable capacitor C36 is connected in a degenerativefeed-back arrangement across the collector and base of Q8 to preventspurious noise from causing Q8 to oscillate. A variable resistor R64 iSconnected in parallel with resistor R63 and is adjusted to keep thecircuit from oscillating unless a signal of predetermined frequency isapplied. A parallel RC network is connected between the emitter and thevoltage supply ZB- and a load resistor R84 is connected between thecollector of transistor Q8 and ground. By adjusting capacitor C36 sothat out-of-phase voltage is fed from the collector to the base oftransistor Q8 in a feedback arrangement, the operation of the circuit isstabilized. The twin-T filter provides feedback 180 out-of-phase which,when added to input coding signals of the proper frequency, permits Q8to oscillate. Thus the filter is so constructed as to amplify thedesired frequency and to reject all other frequency signals. It will beapparent, however, that if coding signals having other than tonefrequency characteristics are used, the filter network 76 would bemodified to provide a suitable response to such signals. The design ofsuch a filter network would depend on the coding signals, and will beapparent to those skilled in the art.

The output of the tone filter 76 is connected through a couplingcapacitor C41 to a three-stage filter amplifier 78. The filter amplifierincludes three transistors Q9, Q10, and Q11, each having a parallelresistance-capacitance network connected between their respectiveemitters and the supply voltage B-. These networks are comprised ofresistors R70, R74, R79 and capacitors C44, C46 and C477 respectively,for load and bypass purposes. Additionally, each stage includes a biasresistor, illustrated at R68, R72, and R77, respectively, connectedbetween its base electrode and the supply voltage B-. Similarly, biasresistors R67, R71 and R76 are connected between the respective baseelectrodes and ground. Load resistors R69, R73 and R73 are connectedbetween their respective collectors and ground. A decoupling capacitorC42 is connected across resistor R67 and R68 of the first stage, while acoupling capacitor C43 couples the collector of transistor Q9 to thebase of transistor Q10. Between the collector of transistor Q10 and thebase of transistor Q11 are two serially-connected clamping diodes 146and 148 in series with a coupling capacitor C45. A resistor R75 of thesame magnitude as resistor R73 is connected between diode 148 andcapacitor C45 to ground. Since R73 and R75 both return to ground, R73 iseffectively in parallel with the series connection of R75 and diodes 146and 148. This arrangement establishes a predeter mined minimum voltagethat must be present on the collector of transistor Q10 before thediodes will become conductive to allow an AC signal to pass to capacitorC45. By adjusting the ratio between R73 and R75, the point at which thediodes conduct may be varied, thus determining the amount of signal thatmust be present on the collector of C before conduction will occur. Thispermits a predetermined minimum signal to be selected, thereby reducingthe response of the circuit to background noise signals, when no signalis being transmitted from the remote station. When diodes 146 and 148 doconduct, they appear to the desired signal as a low resistance path andthus present little or no attenuation of the tone signal.

The output from the tilter amplifier 78 obtained at the collector oftransistor Q11 is applied to a voltage control circuit 80 comprised of avoltage doubler having capacitors C48, C49 and diodes 150, 152 connectedin a wellknown manner. Also included in the voltage control circuit is aT -connected circuit consisting of resistors R80, R81 and a capacitorC50. The output of this filter is coupled through a diode 154, andthrough a stabilization resistor R82 to the supply voltage B-. Theoutput of the voltage control circuit 80 is connected through resistorR83 to the input of a DC amplifier 82, the input comprising the baseelectrode of a transistor Q12.

Transistor Q12 is connected as an emitter-follower amplifier primarilyto provide impedance matching between the voltage control circuit 80 andthe on-of circuit 84. The emitter of Q12 is connected through a parallelnetwork including load resistance 94 and bypass capacitance C57 to thevoltage supply B-. The collector of transistor Q12 is connected toground. A coupling resistor R95 connects the emitter of transistor Q12to the base electrode of the transistor switch Q13 of the on-oi circuit84. The emitter of Q13 is connected directly to the supply voltage B-and its collector is connected through load resistor R96 to ground. Thevoltage appearing at the base of Q12 is amplified and applied to thebase of transistor Q13, switching Q13 to its conductive state andapplying the bias supply voltage B- to the input of the audio amplifier70 by way of line 86. The application of this bias voltage turns theaudio ampliiier on and allows the audio signal received by antenna 20 tobe passed through receiver 22 and to the contacts 98 of relay coil RL1.

The voltage appearing at the collector of transistor Q13 is appliedthrough line 88 to the anti-vox circuit 90 which comprises a transistorswitch Q14. This voltage is applied to the base of Q14 through acoupling resistor R97. The emitter of transistor Q14 is connected toground while the collector is connected to the emitter of transistor Q1of the vox amplier 40 by way ot line 92. When the voltage produced bythe tone signal is applied to the base of transistor Q14, it conducts,and that voltage is applied to Q1 to disable the vox amplifier.

The voltage appearing at the collector of transistor Q13 also issupplied through coupling resistor R98 and line 94 to the time delaycircuit 96. This signal is applied to the base of an input gatingtransistor Q15, the collector of which is connected to the supplyvoltage B-. The

emitter of Q15 is connected through a charging capacitor C58 to groundwhile the base electrode is connected through a bleeding resistor R99also to ground. The emitter electrode is connected through a variableresistor R100 and iiXed resistor R101 to the base of transistor Q16. Thecollector of Q16 is connected to the voltage supply B- through theparallel network of filter capacitor C9 and relay coil RL1 while itsemitter is connected `to ground.

The voltage applied to the base of Q15 through line 94 causes transistorQ15 to become conductive, charging capacitor C58. When the voltageacross C58 has built up to a predetermined value, transistor Q16 willconduct, connecting supply voltage B- across coil RL1 and capacitor C59to cause switch contacts 98 to close. This completes the DC circuit fromthe output transformer (not shown) of the second audio amplifier 72 ofreceiver 16 to the transmission line 26. The completion of this DC patheffectively (i.e., electronically) picks up the telephone receiverhandset at the base station and connects the remote station to telephonesystem 28. No

mechanical lifting of the telephone receiver handset is required toeffect this connection, for the base station transceiver 10 may bedirectly coupled to the telephone receiver circuitry, bypassing thetelephone handset. The delay circuit 96 serves to hold the contacts 98closed even after cessation of incoming signals through the receiver tothe on-ot switch. During the time that Q15 is conductive, capacitor C58becomes fully charged so that when Q15 is cut oft the charge on C58 willbleed off through variable resistors R100, R101 and transistor Q16 toground, maintaining Q16 in a conductive state so that current fiowthrough coil RL1 will hold the contacts 98 closed. Q16 will remainconductive for a period of time of adjustable duration, as determined byresistor R and the size of capacitor C58. This continued energization ofrelay coil RLl maintains the DC path between the second audio amplifier72 and the telephone system so that the telephone receiver remainselectronically pickedup for a period of time after transmission from theremote transceiver has stopped. This eliminates the appearance that thereceiver has been hung-up after each transmission by the remotetransceiver. It should be noted that the closing of contacts 98 is theonly way that the DC circuit for the telephone system can be completed,for capacitor C63 prevents this circuit from being completed through theimpedance matching circuit 32. Contacts 98 thus bypass the DC blockingcapacitor 63 and permit actuation of the central oiiice relays of thetelephone company equipment in the normal manner. It will be apparentthat when C58 has discharged sutiiciently to cut orf Q16, contacts 98will reopen, thus effectively hanging up the telephone receiver. Thetime delay is normally sucient to permit this to occur only at the endof a conversation.

lf the transmissions from the remote transceiver have been pulsed by theoperation of a dial-out mechanism at the remote station, this pulsedsignal will appear at the output terminal of the anti-vox circuit 90 inthe form of a pulsed rather than a continuous coding signal, inasmuch asit is the tone signal which is modulated by the dial mechanism. Thistype of transmission will have the same effect upon the circuitry asabove described, holding the base station in its receive mode, but inaddition will provide a pulsating signal to the dial-out pulser 30through line 100. This signal is applied through a coupling diode and aresistor R120 to the base of a normally-off gating transistor Q17connected in grounded emitter coniiguration lwith its collectorconnected through a resistor R122 to a bias supply. A resistor R121 isconnected between the base of Q17 and ground. Q17 turns on at the end ofeach pulse, and when it becomes conductive a path is provided throughits collector-emitter circuit from the source of supply voltage B- toground. The collector of Q17 is connected through capacitor C70 andresistor R123 to ground, the base of a transistor Q18 being connected tothe junction of C70 and R123. By carefully choosing the parameters ofC70 and R123, a selected pulse repetition rate will cause Q18 to becomeconductive, the RC network acting as a resonant amplifier to providepulses of suhcient amplitude to the base of Q18. Since Q18 only respondsto a selected pulse repetition rate, it will not respond to pulsescaused by pressing the push-totalk button at the remote station. Thecollector of Q18 is connected through a relay coil RL2 to the source ofB voltage providing a current path through RL2 whenever Q18 becomesconductive. Thus, whenever a pulse appears on line 100, Q18 becomesconductive to energize coil RLZ and to open the normally closed relaycontacts 120. Q18 conducts only briefiy, permitting contacts 120 toclose almost immediately, the breaking of the line lbeing of suticienttim-e duration to permit the telephone company equipment at 28 torespond.

If a person having a remote transceiver 12 with a dial similar to thetelephone company dial wished to make a call, he would merely press histransmit button to supply tone coding information to the base station byway of the receiver 16, the frequency sensitive lter 76, the on-offswitching circuit 84 and the time delay circuit 96 to ciose contacts 98and electrically lift the telephone receiver located at the basestation. By holding the transmit button in the transmit position, theanti-voX `circuit 90 would prevent the dial tone present in thetelephone system 2S from changing the condition of the hip-flop 22 andthus prevent the base transceiver from switching to its transmit mode.When the tone coding oscillator in the remote transceiver is pulsed onand off by means of the dial mechanism, contacts 126 will open and closewith each pulse. When open, contacts 12% break both the AC path from thetelephone system 23 through impedance matching circuit 32 to the voxampifier 4u and the DC path to the telephone system dialing equipment.This prevents the dial tone from switching transceiver 11i to thetransmit inode, and causes a normal dialing operation to take place.After the first digit has been dialed, the telephone company equipmentresponds to cut olf the dial tone, thus insuring that the transceiverwill not switch out of its receive mode. if a busy signal. is present onthe line, the dialing operation will be ineffective, and at thecompletion of this operation, the busy signal will come on the line,switching the transceiver' 16 to transmit. Although the presentembodiment is described with respect to a pulsed dialing operation, thesystem is easily adaptable for use with any type of signal required bythe telephone system 28.

As successive digits are dialed at the remote transceiver 12, relay RL?.will open and close contacts 12) to break and make the DC path to thetelephone system automatic dialing equipment until the desired numberhas been dialed. When the subscriber being called answers his telephone,a two-way conversation may then ensue. After the conversation isterminated the time delay circuit controlling contacts 9S will open theDC path and hang up the telephone receiver, making the system ready tohandle another call originating from either the remote transceiver orany subscriber of the telephone system 28.

Before turning to a description of the remote unit, it should be notedthat the above-described system may be used as a wireless radio relaybetween two stations by modifying the time delay network 96 in themanner illustrated in FG. 4, In this usage, the coupling circuitry ofFlG. 2, including the dial-out pulser 30 and the impedance matchingnetwork 32 as well as the telephone system connection 28, is replaced bya conventional transceiver 162, and relay RL1 is used to switch thisstandard transceiver between its transmit and receive modes. Thetime-delay circuit 96 is modified by removing the charging capacitor C58and adding a bias resistor R103 between the base of transistor Q16 andground. Series re sistors R100 and. R101 are lumped together as resistorRHS. Relay coil RLl is used to operate the contacts RC2 and RC3 whichare connected to a transmit-receive switch (not shown) in thetransceiver 162 and generally indicated at T and R. AThe output of thesecond audio amplifier 72 of the base transceiver 10 is connected to theinput terminals 164 which are connected to the audio amplifier stage ofthe transmitter section (not shown) of the transceiver 1o2.

Output terminals 166 of the audio amplifier stage of the receiversection (not shown) of transceiver 162 are connected to the audioamplifier 36 of transmitter 14 and to the vox amplifier 4t) of the basetransceiver 10. Thus, when a tone signal is received from a remotestation such as a remote transceiver 12, the coil RL1 will be energizedto actuate contacts RC2 and RC3 of the standard transceiver 162. Thisturns on the transmitter section of transceiver 162 and permitstransmission by this transceiver of the audio frequency signals receivedby the base transceiver 10. Similarly, when a signal sent by anotherremote stations, which may comprise either a base unit similar totransceiver 16 or another remote unit similar to transceiver 12, ispicked up by the receiver of transceiver 162,

this signal will be transferred to transmitter 14 of base transceiver 10for transmission thereby. Such a system is illustrated in block diagramform in FG. 6. As shown there, the relay link comprises the standardtransceiver 162 connected as above described to a relay station unit 10which is a modified version of the base transceiver 1() shown in FIG. 1and described above. This relay link may be used to relay signals from abase station unit 1l) such as the unit of FIG. 1 connected to atelephone line and a remote station unit 12 such as the hand-held unitillustrated in FIG. 1. It will be apparent that in place of the basestation unit 1th shown here a second remote station unit could be used,or that a plurality of remote units 12', 12 could be used with a singlebase station. Further modifications would include the use of a pluralityof `relay links or the use of a single relay link with the standardtransceiver connected to the relay station 11i' through a longtransmission line. Of course the tone cod ing feature would still beavailable in such a situation, the standard transceiver 162 either beingadapted to pass the tone signals transmitted by the base Station unit 10and the remote station unit 12 or being provided with tone oscillatorsand tone filters, of its own. It will also be apparent that the RF linksbetween the various stations may be of different frequencies, althoughthe use of tone coding in the relay station unit 10 makes the use ofdifferent frequency channels unnecessary.

Turning now to a more detailed description of the remote Station unit12, there is Shown in FIG. 5 a block diagram of a preferred embodimentof such a unit. As has been noted above, the remote station unit ispreferrably a small, portable hand set.

The remote transceiver 12 includes an antenna 170 connected through acoupling capacitor C70, a loading coil 174, RF choke 176 and currentlimiting resistor 178 to ground. The junction of coils 17d` and 176- isconnected through a diode 189, the primary of a coupling transformerT15', line 182, line 184, normally-closed contacts 186 and line 138 tothe negative terminal of a supply voltage battery 1%. The positiveterminal of the battery may be connected through a power on switch 192to ground. By reason of the fact that contacts 186 are normally closed,a current path is provided through diode 180 between ground and thesource of supply voltage, causing diode 180 to conduct and thus to beopen to the passage of incoming signals. This establishes a connectionbetween the antenna and transformer T15 to couple the antenna to thereceiver portion of the remote transceiver. Thus, the remote unit isnormally in the receive mode so that it may respond to any signalstransmitted by the base unit.

The receiver section of the remote unit comprises a mixer and localoscillator 192 coupled to the antenna through transformer T15 andconnected across its secondary winding. The output of the mixer circuitis passed through IF amplifier 1% and is applied to the input ofdetector circuit 196. These circuits are conventional and well-known inthe art and thus are not shown in detail,

The received signal from the base unit would normally comprise an RFcarrier modulated by a tone coding signal and by an audio frequencysignal such as a ringing signal or voice modulation. The output of thedetector circuit 196 will include the tone signal and the audiofrequency modulation. Of course, it will be recognized that the codingsignal may also be an audio frequency signal and is not restricted tofrequencies out of the audio range.

The output of the detector is fed through a variable volume controlresistor R to the input of a tone trap 2110 which attenuates the tonecoding signals and applies the audio frequency signals, either ringingor voice, to the input of the first audio amplified 202. However, thefirst audio amplifier is normally turned off to prevent undesiredsignals such as noise or audio signals from another source not codedwith the proper tone pattern from activating transceiver 12. To turn thefirst audio amplifier 202 on, a tone squelch switch 204 responds to theproper injut tone signal to connect the first audio amplifier to itsbias supply and thus turn it on. The input for switch 204 is obtainedfrom the output of detector 196 by Way of line 2016. The tone switch mayconsist of a highly selective band pass filter of the doube-T typeconnected between the collector and base of a transistor to provide ahighly frequency sensitive oscillator circuit similar to that used inthe frequency sensitive filter 76 of the base station. The output ofsuch an oscillator circuit may then be ampli-fied and used to operate atransistor switch in known manner. Thus, only input signals coded-withthe proper tone freqnencywill operate the tone squelch switch 204 andturn the first audio on to permit audio frequency signals to pass.

The output of first audio amplifier 202, which amplifier is ofconventional construction, is applied to the input of a conventionalmodulator and audio power amplifier 208 for amplification andapplication through a coupling transformer T17 to a loud speaker 210.

In order to provide maximum volume for ringing signals received from thebase station, a bypass switch 212 is provided which, when closed,bypasses the volume control resistor R130 and the tone trap 200, passingaudio frequency and tone signals directly to the input of the firstaudio amplifier 202. The tone signal turns on the first audio by meansof switch 204 in the usual manner and there results a maximum volumesignal output from speaker 210, the output combining the ringingsignals, the voice signals, if any, and the tone signals. Thisarrangement insures a maximum ringing signal to attract the attention ofthe user of the remote transceiver. As a matter of convenience, thebypass switch 212 may be connected to the antenna 170 which may be ofthe telescoping type, switch 212 being closed when antenna 170 istelescoped to its shortest length, as would be the case when thetransceiver is being carried in a pocket or the like.

Upon receipt of a ringing signal, or to respond to a voice signal, theuser of the remote transceiver 12 would press the push-to-talk button212 (or its voice-operated equivalent), opening the normally-closedcontacts 186 and 214, disconnecting the supply voltage from the receiversection of the transceiver and from the Voice coil of speaker 210,disabling both. Button 212 closes contacts 216, 218, 220 and 222 toswitch the remote transceiver to its transmit mode. The closing ofcontact 216 connects-microphone 224 to the source of bias voltage andcompletes a circuit through line 226 to the input of first audioamplifier 202. The closure of contact 218 applies the bias voltage frombattery 190 through line 228 to a tone oscillator 230, turning theoscillator on. The tone oscillator may be of any suitable constructionand serves to generate coding signals of the desired frequency and/ orpattern. The output of tone oscillator 230 is applied through normallyclosed contacts 232 and line 234 to the input of the modulator and audioamplifier circuit 208 to combine the coding signal With the audiosignals from microphone 224.

Normally-closed contacts 232 are a part of a dial mechanism illustrateddiagrammatically at 116. This mechanism is a conventional telephone dialmechanism which serves to open and close contacts 232 in a predeterminedpattern in the manner well known in the arts. This mechanism may be usedto initiate telephone calls from the remote transceiver by pulsing thetone oscillator output to produce tone pulses in the base station andthus to operate the dial-out puiser (FIG. 1) in the manner previouslydescribed. If desired, means may be provided automatically to close thetransmit mode switches upon winding of the dial mechanism.

The closure of contacts 220 upon activation of the push-to-talk button212 applies a bias voltage through line 236 to defeat the tone squelchswitch 204 and turn on the first audio amplifier 202. Closure of thisswitch 212 then permits signals from microphone 224 to pass through theaudio amplifier 202 to the modulator 208.

Closure of switch contacts 222 connects the battery through line 238,the secondary of transformer T17 and line 240 to turn on the RFoscillator 242 and the RF amplifier 244 of the transmit section of theremote transceiver 12. This bias voltage is also applied through thesecondary of a coupling transformer T16, through a diode 246 connectedback-to-back with previously mentioned diode 180, through the RF choke176 and the current limiting resistor 178 to ground. Closure of thiscircuit causes diode 246 to conduct, effectively connecting the RFamplifier 242 through transformer T16 to the antenna M170.cgornpletesthe transfer of the transceiverrfrom its receive to itstransmit mode.

Audio signals from microphone 224 and tone signals from oscillator 230thus may pass through modulator and audio amplifier circuit 208,transformer T17 and line 240 to modulate the RF carrier signalsgenerated by RF oscillator 242 and amplified by amplifier 244. Themodulated RF signals are then transmitted by way of antenna 170. If thetransmission from the remote transceiver 12 is in answer to a ringingsignal, the transmitted modulation would normally be the tone coding anda voice modulation. On the other hand, if a telephone call is beinginitiated by the remote transceiver, the transmitted RF signal would bemodulated by a pulsed tone oscillation. Of course, upon release of thepush-to-talk button 212, the transceiver will immediately revert to itsreceive mode. This permits a two-way conversation to be carried on witha base station, the remote transceiver controlling the direction ofcommunication.

Since this system is designed to be used on a shared channel basis withmany other systems similar to the one described herein as well as othertransceiver combinations of standard communication networks, therenormally will be several conversations carried on at one time on asingle channel. The code toning signal system described herein providesisolation of this system from any other communication using the samechannel and will insure that no audio information except that which issupplied by this system will return to the output of the receiversection of the base unit, whether it be connected to a telephoneexchange system or to a transmission line. By careful selection of tonefrequencies, many systems such as that described herein could share thesame RF channel and yet maintain isolation between each other and othercommunicators using the same channel on a sharedchannel basis.Similarly, the use of coding circuitry in the base unit insures that theremote transceiver will hear only the information transmitted by thebase station which is modulated by the proper coding pattern. It will beapparent, then, that the number of systems used on a single carrierfrequency or channel is limited only by the number of tone frequenciesavailable or by the selectivity of the tone filter used in the system,keeping in mind the physical proximity of the systems and the channelsavailable as well as the possibility of simultaneous use by differentsubscribers. It will also be apparent that the systern is not limited tothe use of a single remote transceiver with a single base station unit,but that any practical number of remote transceivers may be used inconjunction with a single base station unit, provided that all are codedthrough the same frequency or pattern. Similarly, a single remotetransceiver could be used in conjunction with a plurality of basestation units.

Thus there has been provided a relay communication system having manyuses, the foremost of which is its use as a remote telephone extensionwherein a telephone may be answered at a location remote from thephysical location of the telephone receiver. 1t will be obvious to thoseskilled in the art that many modifications and variations utilizing thepri'iciplcs set forth herein `may be devised. However, the scope of theinvention is not limited to the specific embodiments disclosed herein,but includes the various alternatives and modifications that fall withinthe true spirit and scope of the invention as defined by the followingclaims:

I claim:

1. In a remote tele-phone extension system, a first transceiver having afirst transmitter section and a first receiver section; a first antenna;first antenna switch means for selectively connecting said antenna tosaid transmitter section or said receiver section for operation in atransmit or receive mode, respectively, said first transceiver normallybeing in its receive mode; impedance matching means electricallycoupling said first transmitter section to a telephone system; controlcircuit means including monostable circuit means responsive to signalsfrom said telephone system to switch said first transceiver to itstransmit mode, said monostable circuit reverting to its stable stage andreturning said first transceiver to its receive mode during any pausesin said signals from said telephone system; frequency sensitive filtermeans in circuit with said first receiver and responsive to a receivedsignal of predetermined characteristic for energizing first receiveswitch means electrically to couple said first receiver section throughsaid impedance matching means to said telephone system withoutuncoupling said first transmitter section from said telephone system,said frequency sensitive filter means including control circuitdisabling means for preventing said first transceiver from switching toits transmit mode during reception of said signal of predeterminedcharacteristic; holding means including a time delay means formaintaining the energization of said first receive switch means for apredetermined period after the end of said signal of predeterminedcharacteristic and thereafter de-energizing said first switch means touncouple said tirst receiver section from said telephone system, wherebysaid first transceiver is adapted to receive incoming calls from saidtelephone sy-stem and transmit said calls as modulations of a radiofrequency carrier, and is further adapted to receive modulated radiofrequency signals from a remote transmitter, demodulate said signals,and transfer said demodulated signals to said telephone systeni.

L'. The remote telephone extension system of claim 1, further includinga second transceiver remote from said first transceiver and having asecond transmitter section and a second receiver section; a secondantenna; a second antenna switch means for selectively connecting saidsecond antenna to said second transmitter section or said secondreceiver section for operation in a transmit or receive mode,respectively, said second transceiver normally being in its receive modeand responsive to signals transmitted by said first transceiver; andsaid second transmitter section including means for continuouslymodulating by said signal of predetermined characteristic radiofrequency signals transmitted by said second transceiver, whereby saidremote transceiver can receive and answer calls from said telephonesystem.

3. The remote telephone extension system of claim 2 wherein said secondtransceiver further includes dialing means for modulating the signaltransmitted to said first transceiver and means in said firsttransceiver responsive to said dialing modulations vfor activating thedialing circuits of said telephone system, whereby telephone calls canbe initiated by said second transceiver.

4. The remote telephone extension system of claim 2, wherein said firsttransceiver includes means for generating and transmitting a radiofrequency carrier modulated by a signal of said predeterminedcharacteristic, said second transceiver having means for responding tosaid signal of said predetermined characteristic, said last named meansincluding means for preventing reception of signals not having saidpredetermined characteristic.

5. The remote telephone extension system of claim 2, wherein said secondantenna switching means includes a normally-closed circuit for holdingsaid second transi8 ceiver in its receive mode and transmit-receiveswitch means for opening said normally-closed circuit and closing anormally-open circuit, said normally-closed circuit including a firstblocking diode and coupling means for connecting said second antenna tosaid second receiver section, said normally-open circuit including asecond blocking diode and coupling means for connecting said secondantenna to said second transmitter section. Whereby operation of saidtransmit-receive switch converts said second transceiver from itsreceive to its transmit mode.

6. The remote telephone extension system of claim 5, further including asource of direct current connected through said normally-closed circuitto hold said first blocking diode in a state of conduction, wherebyradio frequency signals received by said second antenna can pass throughsaid first diode to said second receiver section, said second diodebeing normally nonconductive to block from second transmitter sectionthe radio frequency signals received by said second antenna, the openingof said normally-closed circuit causing said first diode to becomenon-conductive and the closing of said normallyopen circuit connectingsaid source of direct current through said second diode, `whereby saidsecond diode becomes conductive to pass signals from said secondtransmitter section to said second antenna, the signals from said secondtransmitter section being blocked from said second receiver section bysaid rst. diode.

7. The remote telephone extension system of claim 4, wherein said meansin said second transceiver for responding to said signal of saidpredetermied characteristic includes filter means for separating saidlast-named signal from any other signals carried by said radio frequencycarrier and for applying said other signals to a normally-off audioamplifier, squelch means responsive only to said signal of saidpredetermined characteristic to turn on said audio amplifier wherebysaid other signals may be amplified, and an output transducer forconverting said other signals to sound waves.

8. The remote telephone extension system of claim '7, wherein saidsecond receiver section further includes volume control means and bypassswitch means for bypassing said volume control and said filter means,whereby said signal of said predetermined characteristic and said othersignals may be applied through said audio amplifier to said outputtransducer at maximum amplitude.

9. The remote telephone extension system of claim 7, wherein said secondtransmitter section further includes a transducer for converting soundwaves to audio frequency electrical signals, means operative during thetransmit mode of said second transceiver for turning on saidnormally-off audio amplifier, and means for applying said audiofrequency electrical signals through said audio amplifier to modulatormeans, whereby said audio frequency signals modulate said radiofrequency carrier transmitted by said second transceiver.

10. The remote telephone extension system of claim 7, wherein saidsecond transceiver is normally on to monitor incoming calls, said secondtransceiver including attenuator volume control means for controllingthe amplification of said other signals, said second antenna 'beingretractable, bypass switch means operable by said second antenna and-being closed when said second antenna is retracted to bypass saidvolume control means, whereby the full amplitude of said other signalsis applied to said output transducer.

11. The remote telephone extension system of claim 2, wherein said meansin said second transmitter section for continuously modulating radiofrequency signals transmitted by said second transceiver includesgenerator means for producing said signal of a predeterminedcharacteristic, modulator means for modulating said radio frequencycarrier with said signal of a predetermined characteristic, and meansfor applying said modulated radio frequency carrier to said secondantenna for transmission; said second transceiver further includingdialing means for selectively interruptingsaid signal of saidpredetermined characteristic to produce a dialing modulation of saidradio frequency carrier; and means in said first transceiver responsiveto said dialing modulation for activating the dialing circuits of saidtelephone system, whereby communication between subscribers of saidtelephone system and said second transceiver can be initiated at saidsecon-d transceiver.

12. The remote telephone extension system of claim 11, wherein saidmeans in said first transceiver responsive to said dialing modulationincludes dial-out pulser means interposed between the output of saidreceiver section and said telephone system, said pulser means beingadapted to respond to said selective interruptions in said signal ofpredetermined characteristic to correspondingly interrupt saidconnection between said first receiver section and said telephone systemwithout deenergizing said first receive lswitch means, whereby thedialing circuits of said telephone system are activated.

13. The remote telephone extension system of claim 1, wherein said firstreceiver section includes detector means, the output of said detectormeans being applied to a normally-off audio amplifier and to a filternetwork in said frequency sensitive filter means, said filter networkresponding to a signal of said predetermined characteristic to producean output, said frequency sensitive filter means further includingon-off switching means responsive to an output from said filter networkto turn said audio amplifier on and to disable said control circuitmeans to hold said monostable circuit in its stable state whereby saidfirst transceiver is maintained in its receive mode for the duration ofreception of said signal of predetermined characteristic.

14. The remote telephone extension system of claim 13, wherein saidholding means is responsive to the output of said frequency sensitivefilter means to energize said first receive switch means to connect theoutput of said audio amplifier through said impedance matching means tosaid telephone system, whereby audio frequency signals received by saidfirst transceiver are transferred to said telephone system, said timedelay means maintaining the connection between said audio amplifier andsaid telephone system for a predetermined time after the output fromsaid filter means is cutoff.

15. The remote telephone extension system of claim 1, said firsttransceiver further including dial-out pulser means interposed betweenthe output of said first receiver section and said telephone system andadapted to respond to predetermined modulations of the output from saidfrequency sensitive filter means to interrupt said connection betweensaid first receiver section and said telephone system in accordance withsaid predetermined modulations, without deenergizing said first receiveswitch means, whereby the dialing circuits of said telephone system areactivated to effect a dial-out operation.

16. The remote telephone extension system of claim 1, wherein saidcontrol circuit me'ans further includes normally-off VOX Icircuit meansfor controlling the state of conduction of said monostable circuit, saidsignals from said telephone system turning on said vox circuit means tocause said monostable circuit to assume an unstable state of conductionthereby to disable s'aid first receiver section and to connect saidfirst transmitter section to said first antenna, pauses in said signalsfrom said telephone system permitting said vox circuit means to returnto its normally-off condition, whereby said monostable circuit revertsto its stable st'ate, thereby to disable said lfirst transmitter sectionand to connect said first receiver section to said first antenna.

17. The remote telephone extension system of claim 16, wherein saidfirst antenna switch means includes third and fourth blocking diodesconnected back-to-back, said first antenna being connected to thejunction of said diodes, said third diode being connected throughreceiver section coupling means to said monostable circuit, wherebyconduction of said monostable circuit in its stable state unblocks saidthird diode and couples said first antenna to said first receiversection; said fourth diode being connected through transmitter sectioncoupling means to said monostable circuit, whereby conduction of saidmonostable circuit in its unstable state unblocks said fourth diode andcouples said first antenna to said first transmitter section.

18. The remote telephone extension system of claim 16, w-herein saidcontrol circuit disabling means includes anti-vox circuit means in saidfrequency sensitive filter means, said anti-vox circuit means beingenergized during the reception of said signal of predeterminedcharacteristic to hold said vox circuit in its normally-off condition,whereby said signals from said telephone system cannot turn said voxcircuit means on during the reception of said signal of predeterminedcharacteristic.

19. The remote telephone extension of claim 1, wherein said frequencysensitive filter means includes a filter network responsive only to saidsignal of said predetermined characteristic, whereby said firsttransceiver will receive radio frequency signals from a remotetransceiver only 'when such radio frequency signals are modulated Ibysaid signal of said predetermined characteristic.

20. The remote telephone extension system of claim 4, wherein said firstand second transceivers operate on a single frequency channel forwireless contact with each other, said signal of said predeterminedcharacteristic being a tone coded signal whereby transmissions by eitherof said transceivers will be secure from reception by other similarsystems having different frequency sensitive Ifilter means, and wherebysaid first and second transceivers will not respond to radio frequencysignals on said single frequency channel but not modul-ated by saidsignal of said predetermined characteristic.

21. In a communications receiver having a retractable antenna, saidreceiver being normally on to monitor incoming calls, a detector stagefor separating RF carrier signals from audio frequency signals, an audiostage for amplifying said audio frequency signals for application t0 aloudspeaker means, attenuator volume control means for said audiofrequency signals, and bypass switch means operable by said antenna,said bypass switch being closed when said antenna is retracted to bypasssaid volume control, whereby the full amplitude of said audio frequencysignals is applied to said loudspeaker means.

References Cited UNITED STATES PATENTS 2,442,815 6/ 1948 Kelley 325-5 X2,571,031 10/1951 Hansell 325-392 X 2,875,330 2/1959 Padgett et al.325-312 2,935,605 5/1'960 Mathieu 325-64 X 2,935,606 5/-1960 Harrison etal. 325-16 2,974,221 3/1961 Peth 325--55 X 3,037,114 5/1962 Bier et al.325-370 3,065,421 11/1962 Hart 325-55 3,183,373 5/1965 Sakurai 307--88.513,198,88-8 8/1965' Lemelson 179-41 3,233,109 2/1966 Byles et al.325--406 'X 3,240,879 3/196-6 Bryant 179-41 JOHN W. CALDWELL, PrimaryExaminer'.

DAVID G. REDINBAUGH, Examiner.

B. V. SAFOUREK, Assistant Examiner.

1. IN A REMOTER TELEPHONE EXTENSION SYSTEM, A FIRST TRANSCEIVER HAVING AFIRST TRANSMITTER SECTION AND A FIRST RECEIVER SECTION; A FIRST ANTENNA;FIRST ANTENNA SWITCH MEANS FOR SELECTIVELY CONNECTING SAID ANTENNA TOSAID TRANSMITTER SECTION OR SAID RECEIVER SECTION FOR OPERATION IN ATRANSMIT OR RECEIVE MODE, RESPECTIVELY, SAID FIRST TRANSCEIVER NORMALLYBEING IN ITS RECEIVE MODE. IMPEDANCE MATCHING MEANS ELECTRICALLYCOUPLING SAID FIRST TRANSMITTER SECTION TO A TELEPHONE SYSTEM; CONTROLCIRCUIT MEANS INCLUDING MONOSTABLE CIRCUIT MEANS RESPONSIVE TO SIGNALSFROM SAID TELEPHONE SYSTEM TO SWITCH SAID FIRST TRANSCEIVER TO ITSTRANSMIT MODE, SAID MONOSTABLE CIRCUIT REVERTING TO ITS STABLE STAGE ANDRETURNING SAID FIRST TRANSCEIVER TO ITS RECEIVE MODE DURING ANY PAUSESIN SAID SIGNALS FROM SAID TELEPHONE SYSTEM; FREQUENCY SENSITIVE FILTERMEANS IN CIRCUIT WITH SAID FIRST RECEIVER AND RESPONSIVE TO A RECEIVEDSIGNAL OF PREDETERMINED CHARACTERISTIC FOR ENERGIZING FIRST RECIEVESWITCH MEANS ELECTRICALLY TO COUPLE SAID FIRST RECEIVER SECTION THROUGHSAID IMPEDANCE MATCHING MEANS TO SAID TELEPHONE SYSTEM WITHOUTUNCOUPLING SAID FIRST TRANSMITTER SECTION FROM SAID TELEPHONE SYSTEM,SAID FREQUENCY SENSITIVE FILTER MEANS INCLUDING CONTROL CIRCUITDISABLING MEANS FOR PREVENTING SAID FIRST TRANSCEIVER FROM SWITCHING TOITS TRANSMIT MODE DURING RECEPTION OF SAID SIGNAL OF PREDETERMINEDCHARACTERISTIC; HOLDING MEANS INCLUDING A TIME DELAY MEANS FOR
 21. IN ACOMMUNICATIONS RECEIVER HAVING A RETRACTABLE ANTENNA, SAID RECEIVERBEING NORMALLY ON TO MONITOR INCOMING CALLS, A DETECTOR STAGE FORSEPARATING RF CARRIER SIGNALS FROM AUDIO FREQUENCY SIGNALS, AN AUDIOSTAGE FOR AMPLIFYING SAID AUDIO FREQUENCY SIGNALS FOR APPLICATION TO ALOUDSPEAKER MEANS, ATTENUATOR VOLUME CONTROL MEANS FOR SAID AUDIOFREQUENCY SIGNALS, AND BYPASS SWITCH MEANS OPERABLE BY SAID ANTENNA,SAID BYPASS SWITCH BEING CLOSED WHEN SAID ANTENNA IS RETRACTED TO BYPASSSAID VOLUME CONTROL, WHEREBY THE FULL AMPLITUDE OF SAID AUDIO FREQUENCYSIGNALS IS APPLIED TO SAID LOUDSPEAKER MEANS.